If you need detailed Census information from any time period (1790-present) Social Explorer is the easiest tool to use. It is especially useful for pulling out statistics for the same place over time. If you want to compare the population of a state, county, etc over a long period of time (for example: 1950 to 2010) Social Explorer is the easiest way to do this.
For users who wish to create their own GIS maps using historical Census data, The National Historical Geographic Information System (NHGIS) is an excellent, free source. It contains the core data used to build Social Explorer and users can download the data they need as shapefiles, ready to import into GIS software.
See Social Explorer's Help section for tutorials.
Data.Census.gov is the Census Bureau's official source for accessing current and recent Census data. Data from the 2000 and 2010 Decennial Censuses, plus the American Community Survey, Economic Census and more. A full list of included data can be found here. Older data is only available from the Census Bureau through their FTP site.
Although Social explorer is the easiest way to get basic historical data, it simply does not include all the data. If you're looking for block or block group data, some specific variables or other specialized geographies, the original Census volumes are often the only place to turn. SLU Libraries has all these volumes in print, most held at our Locust Street Facility. The Census Bureau has also digitized a large number of these volumes. Volumes for each decade are available digitally and usually even those that are not digitized are listed under the appropriate decade, so you can get an idea of what is available. If you're not sure what volume you need or what might be available, get in touch with the Government Information Librarian.
Print versus Digital!